1. FIELD OF THE INVENTION
This invention pertains to the field of hydraulic pump controls and more particularly to controls for preventing excessively low pressure and resulting cavitation in the pump.
2. DESCRIPTION OF THE PRIOR ART
An hydraulic pump includes an inlet port, which is supplied with fluid from a reservoir or other source of hydraulic fluid, an outlet port connected to a pressure line, pumping chambers to carry the fluid from the inlet port to the outlet port, and a mechanical drive for activating the pumping chambers. In most hydraulic pumps the pumping chambers increase in size as they rotate toward the inlet, thereby creating a partial vacuum in the chambers. The chambers then decrease in size as they rotate toward and approach the outlet in order to push fluid from the chamber into the system. The vacuum at the inlet is used to create a pressure difference so that fluid will flow from the reservoir into the pumping chambers. However, in many systems the inlet is charged or supercharged, a technique in which a positive pressure, rather than a vacuum, is produced at a pressurized reservoir to create a head of fluid above the inlet. Frequently a low pressure charging pump is used to force fluid into the pumping chambers.
There are many different types of pumps used to pressurize hydraulic systems. Such pumps are generally fixed, constant displacement or variable, adjustable displacement versions. In a fixed displacement pump, the flow rate can be changed only by varying the drive speed. In a variable displacement pump, there is provision for changing the size of the pumping chambers so that the flow rate can be changed by moving the displacement control or changing the drive speed or both of these.
Cavitation is a condition that often occurs in a pump inlet when fluid is supplied at an insufficient rate to supply enough oil to keep the inlet filled. The condition produces in the hydraulic fluid bubbles, which implode as they are exposed to system pressure at the outlet of the pump. In addition to excessive pump speed, other reasons for cavitation include excessive restriction in the inlet line, too low a location of the reservoir level below the inlet, and high viscosity of hydraulic fluid. Ideally there should be no vacuum or even a slight positive pressure at the pump inlet; otherwise, cavitation can result.
Cavitation causes erosion of metal within the pump and increases deterioration of the hydraulic fluid. A badly cavitating pump makes a very distinctive noise as the bubbles implode under pressure.
A positive method to make sure that a pump does not cavitate is to check the inlet with a vacuum guage. Cavitation is prevented by keeping the inlet clean and free of obstructions, by using as large and as short an inlet line as possible with minimum number of bends, and by operating within rated drive speeds.
One way to avoid cavitation in a pump is to pressurize the pump inlet. The easiest way to charge the inlet is by locating the reservoir above the pump inlet. Where this is not possible and acceptable conditions cannot be created otherwise, a pressurized reservoir is often used. An auxiliary pump can be used also to maintain a supply of fluid to the inlet at low pressure. Generally a charging pump is used for this purpose, or a positive displacement pump is used with pressure relief valves set to maintain a desired charging pressure.